In paper editions, the cover often reflects the publisher's opinion of the book - better covers/better artists are not "wasted" on the books that they don't think will sell well; they are reserved for the best-selling authors or the books where they invested a lot of editor's time. Of course, if publishers were perfect, they wouldn't even bother with the unedited dreck they seem to push to fill shelves and would only create best-sellers. Of course, some publishers specialize in formula writers, high volume (and cheap, repetitive covers), knowing that their subscription members don't care about them.
In the Kindle editions, though, the lack of a cover often signals one of two additional classes of books: reformatted (or not) public domain books (that are available free elsewhere and often as well or better formatted) and self-published short stories (which almost never have covers) and books (usually by authors who have either no interest in that part of publishing a book, who are not artistic and can't/don't want to design a cover, or feel the words are the most important). A few, of course, are books where the cover image simply disappeared from Amazon. Of course, in that category of self-published books, there is total garbage (even edited total garbage exists, of course, as most know who've read thru the local bookstore's shelves), racist/political/religious rants (distinguished both by lack of editing and lack of facts, for the most part), run of the mill efforts that some will like, but were rejected by publishers due to lack of marketability and a few gems (just as with music, a gifted writer doesn't HAVE to have a publisher, it just usually increases sales ... just no necessarily profit/income to the writer). Unfortunately, to find these gems in the Amazon sea can be nearly impossible - with no advanced search, you can't exclude the short stories, the public domain reprints, the books with only one review (always a 4 or 5 star review, but also always by the author/publisher), look for a specific price range or length of book.
So, most of us probably do the same thing as at a bookstore - hunt for familiar authors, mock the bestseller list (and especially the illiterate authors that Oprah picks), then browse the shelves by glancing at covers and only reading the fly leaf (summary) of the ones with an interesting cover, then check out a sample chapter of those that sound "interesting" and have a few good reviews. Just that at Amazon, they are reader reviews rather than professional reviews (not necessarily impartial reviews and sometimes even paid for reviews or bad edits of a negative review that make it look favorable). The books with no cover or bland ones get ignored (unless mentioned on social sites like this one).
